1. Field of the Invention
The present invention relates to an electrophotographic image forming method and apparatus, in which a driving current to be supplied to a laser output means is pulse-width-modulated in correspondence with image data.
2. Related Background Art
Conventionally, an electrophotographic image forming apparatus such as a laser printer or the like comprises a semiconductor laser as a laser output means, and supplies a driving current to this semiconductor laser to make it output a laser beam of a corresponding amount. At this time, the laser beam output from the semiconductor laser is modulated in correspondence with image data, and is deflected and scanned in the main scanning direction by a rotating polygonal mirror or the like. At the same time, a photosensitive drum as a latent image carrier means is rotated to move its circumferential surface which is an exposure area in the sub-scanning direction, and the circumferential surface of the photosensitive drum, which is moving in the sub-scanning direction is charged by a charger.
The charged circumferential surface of the photosensitive drum, which is rotating in the sub-scanning direction, is scanned and exposed by the deflected and scanned laser beam to form an electrostatic latent image thereon, which is developed using toner by a developer as a latent image developing means. The developed toner image on the circumferential surface of the photosensitive drum is transferred onto a recording medium by a transfer charger as a toner transfer means, and the toner image transferred onto the recording medium is fixed by a fixing device.
In the above-mentioned image forming apparatus, since a print image is formed by a large number of dots in a matrix, a gradation image can be formed even by binary dots using a dither method or the like. However, since such method lowers the resolution of the print image, an image forming apparatus which forms a gradation image using multi-valued dots is now available.
In such image forming apparatus, for example, when a driving current supplied to the semiconductor laser is pulse-width-modulated in correspondence with image data, the emission time of the semiconductor laser is varied in units of dots. In this case, since each of a large number of dots that form the print image is formed to have a size corresponding to the image resolution, a high-resolution image which is graduated in units of dots can be formed. Furthermore, in such image forming apparatus, since the driving current is adjusted by monitoring the amount of light output from the semiconductor laser, the laser beam amount can be maintained constant.
One prior art of a laser printer as the above-mentioned image forming apparatus will be explained below with reference to FIGS. 1 to 4. Note that FIG. 1 is a schematic block diagram showing principal part of the laser printer, FIG. 2 is a schematic plan view showing the positional relationship among optical parts of the laser printer, FIG. 3 is a circuit diagram showing the internal arrangement of a laser driver and the like, and FIG. 4 is a timing chart showing the relationship among various signals.
As shown in FIG. 1 and the like, a laser printer 1 of this prior art comprises a laser device 2, which integrates a semiconductor laser 3 serving as a laser output means, and a photodiode 4 serving as a laser monitor means. The semiconductor laser 3 outputs a laser beam in correspondence with an input driving current, and the photodiode 4 monitors the laser beam output from the semiconductor laser 3 to output a current signal corresponding to the light amount.
A main body power supply 5 and laser driver 6 are connected to the laser device 2, and they form a current supply means for generating a driving current. A current modulation circuit 7 corresponding to a current modulation means is connected to the laser driver 6, and a data generation means 8 corresponding to a data input means is connected to the current modulation circuit 7.
The data generation circuit 8 comprises a communication I/F (Interface) to which an external apparatus such as a host computer or the like is connected, and externally receives image data defined by a large number of main scanning lines, which are continuous in the sub-scanning direction. The current modulation circuit 7 pulse-width-modulates a driving current supplied to the semiconductor laser 3 in correspondence with image data externally input to the data generation circuit 8 in cooperation with the main body power supply 5 and laser driver 6.
The reflection surface of a polygonal mirror 11 corresponding to a beam deflection means is located on the optical axis of the semiconductor laser 3 of the laser device 2, as shown in FIG. 2, and the circumferential surface of a photosensitive drum 12 serving as a latent image carrier means is located on the reflection optical path of this polygonal mirror 11 via, e.g., a correction optical system (not shown) such as an f-.theta. lens and the like.
The polygonal mirror 11 is rotatably axially supported by a scanner motor (not shown), and deflects and scans a laser beam output from the semiconductor laser 3 in the main scanning direction. The photosensitive drum 12 is rotatably axially supported by a drum driving mechanism (not shown) as a sub-scanning means, to relatively move its circumferential surface, which is scanned and exposed by the laser beam, in the sub-scanning direction.
A BD (Beam Detect) sensor 13 as a beam detection means is placed at a position that leads the photosensitive drum 12 in the main scanning direction within the scan range of the polygonal mirror 11. The BD sensor 13 detects the laser beam deflected and scanned by the polygonal mirror 11 immediately before the laser beam reaches the photosensitive drum 12.
As shown in FIG. 3, an operation control circuit 15 is connected to the BD sensor 13 via an amplifier 14, and is connected to the current modulation circuit 7 and laser driver 6. The operation control circuit 15 comprises, e.g., a microcomputer, and serves as various means when an appropriate control program is installed as its software.
For example, the operation control circuit 15 serves as an exposure control means and output control means when it controls the operation of the current modulation circuit 7 in correspondence with the laser beam detection timing of the BD sensor 13. In this case, as shown in FIG. 4, the operation control circuit 15 controls the semiconductor laser 3 to continuously output a laser beam at a timing the laser beam deflected and scanned by the polygonal mirror is detected by the BD sensor 13, and controls the current modulation circuit 7 to start pulse width modulation of the driving current units of main scanning lines of image data a predetermined period of time after the beam detection timing of the BD sensor 13.
Also, the photodiode 4 of the laser device 2 is connected to the operation control circuit 15 via an amplifier 16. In this case, the operation control circuit 15 serves as a detection control means, and controls the photodiode 4 to detect the laser beam continuously output from the semiconductor laser 3.
The laser driver 6 comprises a laser driving circuit 17 and an APC (Automatic Power Control) circuit 18 corresponding to a current adjustment means. The laser driving circuit 17 is connected to the semiconductor laser 3 in the laser device 2 and to the current modulation circuit 7, and supplies a driving current, which is modulated in correspondence with image data, to the semiconductor laser 3.
The laser driving circuit 17 comprises an analog switch 21, current buffer transistor 22, resistor 23, operational amplifier 24, and the like. The analog switch 21 comprises, e.g., a CMOS (Complementary Metal Oxide Semiconductor) or the like, that is capable of high-speed operation, and turns on/off the semiconductor laser 3 in response to a control signal supplied from the current modulation circuit 7.
The current buffer transistor 22 and resistor 23 are connected in series with the analog switch 21, and the operational amplifier 24 is connected to the base of the current buffer transistor 22, whose emitter is feedback-connected to the negative input terminal of the operational amplifier 24.
The photodiode 4 in the laser device 2, the operation control circuit 15, and the current modulation circuit 7 are connected to the APC circuit 18, which adjusts the driving current generated by the laser driving circuit 17 in correspondence with the light amount detected by the photodiode 4 under the control of the operation control circuit 15.
The APC circuit 18 comprises a comparator 25, constant voltage power supply 26, analog switch 27, hold capacitor 28, and the like. The hold capacitor 28 is connected to the positive input terminal of the operational amplifier 24 in the laser driving circuit 17. The photodiode 4 in the laser device 2 and a resistor 29 are connected to the negative input terminal of the comparator 25, and the constant voltage power supply 26 is connected to the positive input terminal thereof.
The comparator 25 compares the detected voltage input from the photodiode 4, and a reference voltage generated by the constant voltage power supply 26, and outputs the comparison result as high and low signals. The analog switch 27 connects/disconnects the comparator 25 and hold capacitor 28 in response to a sample & hold signal input from the operation control circuit 15.
When the hold capacitor 28 is connected to the comparator 25 by the analog switch 27, it holds a voltage corresponding to the output signal from the comparator 25; when the hold capacitor 28 is disconnected from the comparator 25 by the analog switch 27, it applies the hold voltage to the operational amplifier 24 in the laser driving circuit 17.
Note that the operation control circuit 15 outputs a sampling signal to control the analog switch 27 in the APC circuit 18 to connect the hold capacitor 28 to the comparator 25 at a timing the laser beam deflected and scanned by the polygonal mirror 11 hits the BD sensor 13. On the other hand, the operation control circuit 15 outputs a hold circuit to control the analog switch 27 to disconnect the comparator 25 from the hold capacitor 28 at a timing the deflected and scanned laser beam strikes the photosensitive drum 12.
Various devices such as a charger as a carrier charging means, a developer as a latent image developing means, a transfer charger as a toner transfer means, and the like are placed around the photosensitive drum 12 in addition to the above-mentioned laser scanning mechanism, although their illustration and description will be omitted since their structure is known to those who are skilled in the art. Also, a convey path of a print paper sheet as a recording medium is formed in the gap between the transfer charger and photosensitive drum 12.
The laser printer 1 with the above-mentioned structure can form an image by electrophotography. In this case, a driving current supplied from the main body power supply 5 and laser driver 6 to the semiconductor laser 3 in the laser device 2 is modulated by the current modulation circuit 7 in correspondence with image data externally input from the host computer or the like to the data generation circuit 8.
That is, since the current modulation circuit 7 controls ON/OFF of the analog switch 21 in the laser driving circuit 17 of the laser driver 6 in correspondence with image data, a laser beam that has been modulated in correspondence with the image data is emitted from the semiconductor laser 3.
The laser beam emitted from the semiconductor laser 3 in correspondence with the image data in this way is deflectively scanned in the main scanning direction by the rotating polygonal mirror 11, and is irradiated on the circumferential surface of the photosensitive drum 12, which is rotating in the sub-scanning direction, thus forming an electrostatic latent image as a large number of main scanning lines there.
At this time, since the deflectively scanned laser beam is detected by the BD sensor 13 immediately before it reaches the polygonal mirror 11, the start positions of a large number of main scanning lines, which are continuous in the sub-scanning direction, can be aligned by effecting the emission of a laser beam corresponding to the image data after a predetermined period of time has passed after beam detection by the BD sensor 13.
Since the detection of the laser beam by the BD sensor 13 is required prior to the image scanning, the semiconductor laser 3 continuously emits a laser beam under the control of the operation control circuit 15 at a timing when the deflectively scanned laser beam is irradiated on the BD sensor 13.
In this laser printer 1, when the semiconductor laser 3 continuously emits the laser beam to the BD sensor 13, as described above, the amount of light emitted from the semiconductor laser 3 is detected by the photodiode 4 using the continuously emission laser beam, and the APC circuit 18 in the laser driver 6 adjusts the driving current.
In this case, since the comparator 25 in the APC circuit 18 compares the voltage detected by the photodiode with the reference voltage output from the constant voltage power supply 26, the operation control circuit 15 controls the analog switch 27 in the APC circuit 18 to connect the comparator 25 to the hold capacitor 28 and controls the hold capacitor 28 to hold a voltage corresponding to the amount of light output from the semiconductor laser 3 at a timing the semiconductor laser 3 continuously emits the laser beam.
At a timing the semiconductor laser 3 emits the laser beam to the photosensitive drum 12, the operation control circuit 15 controls the analog switch 27 in the APC circuit 18 to disconnect the comparator 25 from the hold capacitor 28. Hence, the laser driving circuit 17 supplies a driving current corresponding to the voltage held by the hold capacitor 28 to the semiconductor laser 3.
Since the laser beam is pulse-width-modulated in correspondence with image data, the aforementioned laser printer 1 can form a dot-matrix print image which is gradated in units of dots. At this time, since the driving current is adjusted to make the amount of the laser beam continuously emitted from the semiconductor laser 3 constant, thus forming a high-quality image which is gradated in units of dots.
In the image forming apparatus such as the above-mentioned laser printer 1 or the like, a laser beam is pulse-width-modulated in correspondence with image data so as to form an image which is gradated in units of dots, and the amount of light continuously emitted from the semiconductor laser 3 is adjusted to become constant so as to maintain high image quality of a grayscale image.
However, in the image forming apparatus such as the above-mentioned laser printer 1 or the like, in order to maintain high image quality of a grayscale (gradation) image, the pulse width of pulse width modulation must also be appropriately adjusted. Hence, the pulse width of the laser printer 1 is adjusted in, e.g., the final stage of the conventional manufacturing process before the product is delivered.
For this reason, as shown in FIG. 1, the current modulation circuit 7 includes a variable resistor in which maximum and minimum pulse widths are variably set. By adjusting this variable resistor, the pulse width upon modulating the laser beam in correspondence with image data is appropriately set.
In this case, a dedicated light-receiving sensor (not shown) is placed at the position of the photosensitive drum 12, and predetermined adjustment data is input to the data generation circuit 8 in the same manner as image data. In this way, laser beams, which have been modulated to have various pulse widths, are emitted, and their amounts are detected. Then, the pulse width is adjusted to obtain an appropriate light amount.
Upon adjusting the pulse width in this fashion, the light-receiving sensor is placed at the position of the photosensitive drum 12. However, since the position of the light-receiving sensor in the main scanning direction is arbitrarily determined by the operator, it varies in the main scanning direction from one operator to another. In the aforementioned laser printer 1, the amount of the continuously emitted laser beam is monitored by the photodiode 4 immediately before a main scan, and the hold capacitor 28 holds a voltage to obtain an appropriate driving current of the semiconductor laser 3. However, the voltage held by the hold capacitor 28 varies due to charges leaking from the operational amplifier 24 during the main scan.
For this reason, since the position of the light-receiving sensor varies in the main scanning direction from one operator to another, the measurement condition of the amount of the pulse-width-modulated laser beam varies depending on operators, and the pulse widths of laser printers 1 mass-produced cannot be appropriately and uniquely adjusted. For example, a jig or the like can be designed to locate the light-receiving sensor at an identical position. Even in this case, the dedicated light-receiving sensor must be placed at the position of the photosensitive drum 12, resulting in cumbersome operations.
Furthermore, since the aforementioned adjustment is done only before the product is delivered, changes resulting from environments or as a function of time after product delivery cannot be coped with. For example, such adjustment may be done at the time of maintenance by a service engineer. However, in this case, daily changes resulting from environments cannot be coped with in real time.
In addition, upon measuring the amount of the pulse-width-modulated laser beam, the light amount may be erroneously measured due to mixed noise, and the pulse width may be adjusted to a wrong value. Such case can be coped with when the operator repeats light amount measurement to select an appropriate measurement result. However, the load on the operator increases unwantedly.